1. Field of the Invention
The present invention relates to light sources for use in optical communication and optical information systems, more specifically to a semiconductor laser device of the refractive index guide type, operating with a low threshold current and stable fundamental transverse-mode.
2. Descriotion of the Prior Art
FIG. 2. shows in cross section an example of an AIGaAs semiconductor laser device's structure according to the prior art.
ln the first growth step a p-type Al.sub.y Ga.sub.1-y As cladding layer 12 and an n-type GaAs current blocking layer 13 are grown on top of a p-type GaAs substrate 11 using the MOCVD method. An inverted trapezoidal groove 14 is formed by selectively etching into one section of the n-type GaAs current blockinq layer 13. Next, in the second qrowth step, a p-type Al.sub.y Ga.sub.1-y As cladding layer 15, an Al.sub.x Ga.sub.1-x As active layer 16 with an active region 21, an n-type Al.sub.x Ga.sub.1-y As cladding layer 17, and an n-type GaAs contact layer 18 are grown successively. 19 is a negative electrode; 20 is a positive electrode.
In order to cause laser oscillation it is necessary to apply a forward voltage between the negative electrode 19 and positive electrode 20, and also to cause in the acative region 21 a forward current flow greater than the threshold value. In a semiconductor laser device of this structure, the current path is restricted near the groove 14 and the current is made to converqe in the active region 21 by means of the n-type GaAs blocking layer 13. In addition to the double heterojunction in the vertical direction, the Al.sub.x Ga.sub.1-x As active layer 16 is bent to have a refractive index distribution in the horizontal direction. As a result of these special features, the electric current, carriers, and light are efficiently confined almost completely in the active region.
This semiconductor laser device has excellent performance: the low oscillation threshold current being approximately 30 mA for room temperature CW operation for example, stable fundamental transverse-mode operation in temperatures over 90.degree. C., smoothly shaped em-ssion pattern, and small astigmatisms. The above mentioned semiconductor laser device according to the prior art is impossible to make unless the said crystal growth process is carried out in the aforementioned order. In other words, in the first growth step, after the p-type Al.sub.y Ga.sub.1-y As cladding layer 12 and the n-type GaAs blocking layer 13 are grown, the crystal is taken out of the growth furnace. Then the growth of the groove 14 that becomes the current path is carried out. Then the layers from the p-type Al.sub.y Ga.sub.1-y As cladding layer 15 to the n-type GaAs contact layer 18 are grown successively. The wafer is therefore exposed to the air once while being manufactured. At that time, the crystal surfaces exposed to the oxygen and moisture carrying atmosphere naturally begin to oxidize and deteriorate. Because during the time of the second growth step, an inferlor growth is established, it is not possible to grow a high quality crystal and it becomes very difficult to reproducibly realize excellently performing semiconductor laser devices. ln order to solve this, usually, right before the second growth step, the crystal is etched to eliminate the oxidation layer and the like, cleaning the crystal surfaces. However, it is so difficult to clean perfectly that even using the comparatively easy MOCVD method frequently creates problems.
Not just limited to the shown example structure, but with almost all the semiconductor laser device structures having a blocking layer and a refractive index guide, whether using MOCVD or LPE crystal growth methods, it is necessary to carry out a second qrowth step. The main problem point is that the reproducibility of this semiconductor laser device with outstanding qualities has been very low.